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Modern electrical

machines

Copyright 2019 Matrix TSL Limited

Modern Electrical Machines

Modern electrical

machines


Contents

Health and safety 3

Key Relationships 4

Worksheets 5

Worksheet 1 - Getting started 6

Worksheet 2 - Understanding Torque and mass 7

Worksheet 3 - Understanding the DC power supply 8

Worksheet 4 - DC motor characteristics 1 9

Worksheet 5 - DC motor characteristics 2 12

Worksheet 6 - Permanent magnet generator (Dynamo) 14

Worksheet 7 - Shunt and series motor basics 16

Worksheet 8 - Shunt wound motor characteristics 20

Worksheet 9- Series wound motor characteristics 22

Worksheet 10- Universal motor 24

Worksheet 11 - Comparing shunt and series motors 26

Worksheet 12 - Single-phase induction motor 27

Worksheet 13 - Single-phase induction motor - cap start 28

Worksheet 14 - Phase shift in induction motors 30

Worksheet 15 - Variable frequency drive strategies 32

Worksheet 16 - Three-phase motor characteristics 36

Worksheet 17 - Three-phase motor drive strategies 38

Worksheet 18 - Understanding slip 41

Worksheet 19 - Brushless DC motor 43

Worksheet 20 - Three-phase generator 45

Machine information 47

Package information 62

Software information 70

Instructor Guide 82

Equipment Checklist 90

Trouble shooting 91

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Health and safety

Safety

During the design of this product we have paid

considerable attention to the potential risks of

studying electric motors. We believe that we

have come up with the safest possible design.

However there are still some risks that you

need to be aware of. This page shows how we

have considered each danger. You need to

read this and make sure that your students are

protected whilst using the equipment.

Electric shock

This is minimal: the output from the control

box is limited to 24V AC or DC

The dynamometer is capable of generating DC

voltages. At maximum speed, around 3,000

r.p.m., the generated voltage is less than

30VDC.

The control unit will not generate power until

a motor is plugged into the dynamometer. This

prevents the use of third party motors with

the system.

Physical shock

The equipment is heavy. As with other heavy

lab. equipment, if a student drops a device on

his foot, it could cause considerable damage.

You need to decide on the level of responsibil-

ity that students take here.

You can reduce risk by having a technician lay

the equipment out on benches and ensuring

that students are seated at desks whilst using

the equipment.

Exposed rotating parts create hazards as hair

and clothing can get caught in them. The use

of relatively low-power motors reduces the

risk. The plastic guard between dynamometer

and motor under test means that no rotating

parts are exposed..

Caution:

Do not use a PC-based oscilloscope

with the equipment.

Earth loop currents may flow between

its earth connection and the control

box’s earth connection.

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Calculating torque

Torque is the turning effect of a force.

Strictly speaking, the balance used in the investigations measures mass, the quantity of matter in a

body, not force. The balance reads mass in either grams or kilograms. Force and mass are linked by:

Force = mass x acceleration or F = m x a, where a is usually the acceleration due to gravity, 9.81 m /s2.

This allows us to calculate the force pressing down on the balance, generated by the dynamometer.

Torque = force x distance from the axis of rotation. It is measured in newton-metres (Nm).

Combining these equations:

Torque, T = m x a x r where, here, r is the distance of the pressure point from the centre of the machine.

Putting in the value of r for the dynamometer used in this kit:

torque (in Nm) = balance reading (in kg) x 9.81 (m/s2) x 0.03812 (m).

Calculating power

Power is the rate at which energy is delivered and is measured in watts (W).

For a rotating body:

Mechanical power = (2 x n x π x T) / 60 where n is the speed of the motor in r.p.m.

For a DC motor:

Electrical power supplied to the motor = VIN x IIN where VIN = input voltage and IIN = input current.

For a single phase AC motor:

Electrical power supplied to the motor = VIN x IIN x cos(ϴ) where VIN is the AC rms input voltage, IIN is the rms input current and cos(ϴ) is the power factor where ϴ is the phase angle between VIN and IIN in degrees.

For a three phase AC induction motor:

Electrical power supplied to the motor = √3 x electrical power supplied to one phase

Calculating efficiency

Efficiency (η) = (Mechanical power delivered / Electrical power supplied) x 100%

Calculating synchronous speed and slip

The speed of the motor, n, in r.p.m = supply frequency, f, x 60 i.e. n = f x 60.

Synchronous speed in r.p.m = (f x 60 ) / (number of pairs of poles)

(Number of pairs of poles relates to how the motor windings are configured:

a 2-pole motor has one pair of poles, a 4-pole motor has two pairs of poles etc.)

Slip = ((synchronous speed - slip speed ) / synchronous speed ) x 100%

Key relationships

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Worksheets

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Worksheet 1 Getting started

Motors vary hugely in size, shape and technology.

Within a family of motors, for example DC motors,

the characteristics of the machines are very similar

- independent of their physical size and operating

voltage.

This section looks at setting up the equipment.

Then the testing can begin.

Over to you:

1) Connect the control unit to the mains power

using the IEC connector. The quad 7-segment

display and Power LED should light up.

2) Connect the dynamometer to the control

unit with the 25 way D-type lead.

3) Make sure that the software applications and

control unit driver are correctly installed.

When the control box is not connected to the

PC using the USB lead, the box is in manual

mode - the switches and knobs on the front

panel determine the control box parameters.

Now you are ready to test the set up .

Set up a machine test

1) Line up the DC motor in the grooves of the

dynamometer test bed.

It should slide in easily. You may have to un-

do the knurled nuts on the motor bed.

2) Make sure that the rubber ‘spider’ which

buffers the dynamometer and motor under

test is in place. The system will work without

this - it will just be noisy.

(On first use you might have to push hard!)

3) Make sure the motor and dynamometer

couplings are aligned before pushing the

motor home.

4) As you push home the DC motor into the

dynamometer test bed you should hear the

safety microswitches in the dynamometer

cradle click.

5) Tighten the knurled lock nuts to make sure

that the DC motor is secured in place.

6) Insert the balance and remove both

protective plastic covers.

You are now ready to start experimenting

with the equipment, in manual mode.

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Worksheet 2 Understanding torque and mass

DC motors use a permanent magnet in the stator

and direct current powered windings in the rotor.

A commutator is used to reverse the direction of

the current in the rotor to maintain the force on it

as the motor turns.

The graphic shows a very simple commutator. In

practice, the ring has several splits in it, allowing

several rotor circuits to provide more continuous

torque.

Over to you:

With the control box in manual mode:

1) Put the balance under the swing arm and

turn it on.

2) Turn the control unit potentiometers to min-

imum ( fully anti-clockwise.)

3) Slowly increase the value of DC1 until the DC

motor starts to turn.

4) Is there a reading on the balance?

If not, swap over the positive and negative

connections on the motor to change the

direction of rotation.

5) With DC1 potentiometer in the middle of its

range, note of the balance reading and motor

speed in the table below. (The motor speed

will vary a little - estimate an average.)

6) Turn the DC1 PSU pot to increase the voltage

slightly. Measure and record the new balance

reading and speed.

7) Referring to the ‘Key Relationships’ (page 4,)

use the readings to calculate the torque for

the two settings and record your answers .

So what?

To change the direction of a DC motor you

reverse the electrical connections.

The swinging-arm dynamometer can rotate in

either direction. The torque produced is the

same for both. When rotating counter-

clockwise, pressure is applied to the balance.

When rotating clockwise, the pressure is on

the load cell at the back of the dynamometer.

Speed Balance torque

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Worksheet 3 Understanding the DC power supply

The permanent magnets occupy a small space in a DC

motor compared to the space needed for the windings

in motors where the magnetic field is supplied by current-

carrying coils. As a result, DC motors can be physically

quite small for their power rating.

The food blender in the photograph uses a DC motor.

A motor can be controlled by varying the DC

voltage, and hence the power, supplied to it.

Over to you

1) Adjust the DC1 potentiometer so that it is in

about a quarter way of the way round.

2) Use an external oscilloscope, not a PC-based

one, to look at the signal from DC1.

3) Use the upper graph opposite to sketch the

waveform of this signal.

4) Turn the DC1 potentiometer about three-

quarters of the way round and draw the

resulting waveform on the lower graph.

So what?

Modern DC motor power supplies do not vary

the output in an analogue fashion, but use

pulse-width modulation or PWM. In this, the

mark : space ratio of the output is varied.

In the control unit, a bank of FET’s deliver a

PWM signal generated by a computer. This can

be used with DC, with single-phase and with

three-phase motors.

Do not use a PC-based oscilloscope.

DC1 - one-quarter max.

DC1 - three-quarters max.

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Worksheet 4 DC motor characteristics 1

Over to you:

1) Set up the circuit shown above. Connect the

DC motor to the control unit’s DC1 power

supply V+ and 0V terminals.

2) Energise the DC motor to put pressure on

the dynamometer load cell rather than the

balance. (If necessary, reverse the power

supply connections .)

3) Connect the dynamometer to the load

terminals on the control unit to set a load

across the dynamometer.

4) Connect the control box to your computer

using the USB lead.

5) On the computer, load the ‘Open Control DC’

program and ‘RUN’ it.

6) Set the PWM DC output to around 30%.

7) Give the DC motor a small load of around

50% on the dynamometer.

Different types of motor have different

characteristics, such as top speed, torque

at different speeds , voltage and current

ratings, power output and efficiency.

Photo: these tiny DC motors are used in

small handheld devices like toys.

8) Investigate the effect of varying the PWM

voltage between 30% and 100%, taking six

readings across that range. In the table on

the next page record the speed, torque, and

DC power coming from the power supply.

9) Use the information in the ‘Key Relationships’

section on page 4 to calculate the mechanical

power delivered to the dynamometer.

10) In addition, calculate the efficiency of the

motor at each of the six settings .

11) On the graph paper provided, use these

readings to plot a graph to show how the

torque (y-axis) varies with speed (x-axis).

So what?

Each motor has characteristics that make it

suitable for particular applications in industry.

In this learning package, you meet a variety of

motor types and learn to appreciate their

characteristics.

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Speed Torque Electrical

power

Mechanical

power

Efficiency Voltage

Variation of torque with speed

0 2500 Speed (rpm)

Torque(mNm)

0

80

40

1000 500 1500

20

60

100

2000

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Over to you

Describe some of the characteristics of the DC motor.

What speeds give maximum and minimum torque?

At what speed is it most efficient?

When would you use a DC motor as opposed to a different type?

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Over to you:

1) Set up the circuit , connecting the DC motor

to the control unit’s DC1 power supply .

2) Connect the dynamometer to the load

terminals on the control unit to set a load

across the dynamometer.

3) Connect the control box to the computer

using the USB lead.

4) On the computer, run the program ’Open

LogDC.bat’.

) Set the PWM DC output to around 50%.

6) Press the ‘Sweep’ button to run an automatic

speed / torque log.

7) Repeat step 6 for DC outputs of 40%, 60%,

70%, 80% and 90%

8) The program logs the results into an Excel file

Its name is given in the ‘Properties’ section on

the right of the screen. The program is stored

in the ‘Logs’ subdirectory.

When the sequence is finished, load the

results into Excel and use it to plot accurate

speed torque graphs of the motor.

9) On the next page, sketch the results.

So what?

When looking at a motor’s specification to

check its suitability for a particular application,

you often see a family of graphs showing its

performance for different driving conditions.

Understanding this information helps you to

make the right choice.

Fairground ‘dodgem’ cars use simple DC electric

motors operating at between 12 and 48V.

The vehicles have two brushes - one touching

the metal floor ,for 0V, and the other touching

the metal ceiling, at a positive voltage.

Speed control is usually simple on-off.

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Effect of motor speed on torque for different values of

DC power supply voltages

0 2500 Speed (rpm)

Torque(mNm)

0

80

40

1000 500 1500

20

60

100

2000

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The electrical machine in the dynamometer has

exactly the same characteristics as the DC motor.

The only difference is physical - the drive shaft is

longer.

The photograph shows a bicycle dynamo, used to

power electric lights on the front and rear of the

bike.

This worksheet examines the properties of a

permanent magnet generator or ’dynamo’,

using the dynamo found in the dynamometer.

Over to you:

1) Set up the system shown in the diagram.

2) On your PC, run the program ’Open Con-

trol_DC.bat’.

3) Set the dynamometer load to 80%.

4) Use the program to take measurements of

power out, speed, and torque generated by

the dynamo for DC motor driving voltages of

40%, 50%, 60%, 70%, 80% and 90%

5) Use the formulae in the ‘Key Relationships’

section on page 4 to work out the mechanical

power delivered into the dynamometer.

6) Calculate the efficiency of the conversion

from mechanical to electrical power in the

dynamometer.

7) Use the results to plot a graph, using the axes

on the next page.

So what?

The efficiency of an electrical machine changes

with its speed. Choosing the right combination

of machine and gearbox for an application will

help to get optimal efficiency.

Worksheet 6 Permanent magnet generator (dynamo)

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DC motor drive

Torque Dyno power

Dyno efficiency

Speed Mechanical power

40%

50%

60%

70%

80%

90%

Worksheet 6 Permanent magnet generator (Dynamo)

Variation of efficiency with speed for a dynamo with a load of 80%.

0 2500 Speed (rpm)

Efficiency

0

80%

40%

1000 500 1500

20%

60%

100%

2000

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Worksheet 7 Shunt and series motor basics

Wound stator DC motors are often configured as

either shunt wound or series wound.

The separate connections for the field windings of

the stator and rotor / armature allow us to control

the current in both parts of the motor separately,

in order to examine the motor’s behaviour.

The photograph shows an old shunt wound motor.

Over to you:

1) With both the series and shunt motors dis-

connected form the test rig, use a digital mul-

timeter to measure the resistance of the field

windings on both the shunt and series mo-

tors.

2) Use the Ohm’s law formula, I= V/R, to calcu-

late the theoretical field currents when the

field windings are connected to a 24V DC

supply.

3) Repeat the resistance measurement for the

armature windings

4) Record your results in the table opposite.

5) In practice, you get different values of cur-

rent which we discuss below.

Motor Resistance

in Ω

Current

in A

Shunt

Series

Field windings

Armature windings

Red and black terminals are for the armature winding, yellow and purple terminals

for the rotor windings.

Motor Resistance

in Ω

Current

in A

Shunt

Series

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So what?

Electrical machines are made up of multiple windings. When DC flows through them, their electrical

characteristics can be predicted using the Ohm’s law formula. However, once the electrical machine

is running the rotating magnetic fields in the machine alter their behaviour drastically.

The equivalent circuit of the machine then becomes more complicated. In particular, the rotating

magnetic components in the machine induce an electromagnetic force (e.m.f.) which acts to oppose

the flow of current in the windings. You can see the equivalent circuit below.

Understanding how to calculate this opposing e.m.f. is outside the scope of this learning package -

but you need to be aware that it exists.

The reverse e.m.f. increases as speed increases. When a machine stalls, therefore, the reverse e.m.f.

is reduced. The current flowing then increases significantly, increasing the risk of burning out the

motor .

Equivalent circuit of a simple DC machine.

Over to you:

1) Connect the multimeter to the rotor winding of the Shunt motor on the ohmmeter setting.

2) Slowly turn the motor shaft. Explain what you see on the meter.

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With all wound-stator DC motors, you swap

over the connections to either the stator or

rotor windings to make the motor rotate in the

opposite direction.

Reversing the voltage to both sets of windings

keeps the motor rotating in the same direction,

as both fields are reversed.

Over to you:

1) Set up the shunt motor as in the top diagram.

2) On your PC, load the ‘Open Control_DC.bat’

program. Set the output on DC1 ( the rotor)

to 100% or 24V. Set the output on DC2 (the

field coils) to around 70% and press ‘RUN’.

3) Increase DC2 in steps of 10% up to 100%.

In the table on the next page, record the

voltages and currents at each stage.

4) Now, change to the series motor and wire it

up as in the lower diagram.

5) Set the output on DC1 to 40% and press RUN.

6) Increase DC1 in steps of 5%. Again, record

the output voltages and currents each time.

The currents eventually gets so high that the

control unit cuts out.

So what?

This experiment shows that the speeds of the

series and shunt motors are very different for

similar voltage inputs .

The control unit cuts out at a certain speed

(current) for the series motor. The current is

limited to protect the motor and the control

unit.

The control unit delivers an output power up

to 100W. It cuts out when this is exceeded and

shows an ‘Err’ message on the front of the

unit. The software application stops.

Note: the circuits are similar but not quite the same.

Shunt wound

Series wound

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DC1 % DC2 % I1 in A I2 in A Speed in RPM

100 70

100 80

100 90

100 100

Shunt motor

Series motor

DC1 % I1 in A I2 in A Speed in RPM

40

45

50

55

60

65

70

75

80

85

90

95

100

Indicate clearly the point at which the control unit cuts out.

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Worksheet 8 Shunt-wound motor characteristics

Speed control is an important element of

electromechanical system design.

Some motors are better than others at self-

regulating speed. The shunt motor is really

good at speed regulation and for this reason

they are often used in devices like lathes,

which need to run at a constant speed.

Over to you:

1) Set up the shunt motor as shown above.

2) On your PC, run the program ’Open

Log_DC.bat’.

3) Open the program. Set the DC1 output to

100% to power the armature and the DC2

output to around 50% for the field winding .

4) Run the program and press ‘Sweep’ to take

an automatic set of readings for a range of

dynamometer loads.

5) Repeat steps 4 and 5 for field winding DC

outputs of 60%, 70%, 80% and 90%

6) When the sequence has finished load the

results into Excel and use Excel to plot a

graph of speed and torque.

7) On the next page, use the axes provided to

sketch the results.

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Worksheet 8 Shunt wound motor characteristics

The effect of speed on torque for different values of field coil voltage.

Over to you:

1) With the same equipment set up, load the program ’Open Control_DC.bat’

2) Run the program and set the DC output for the armature to 100% and the output for the field

winding to around 50%.

3) Complete the table to show speed, output power, dynamometer power and efficiency.

0 2500 Speed (rpm)

Torque(mNm)

0

80

40

1000 500 1500

20

60

100

2000

Field winding

power supply

Power

out

Dyno

power

Efficiency Speed

rpm

50%

60%

70%

90%

100%

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Worksheet 9 Series-wound motor characteristics

A series-wound motor has the highest torque on

start-up of any motor. However, its speed varies as

the load varies. It is ideally suited to applications

such as winches and cranes where the initially high

torque is important.

The photograph shows a typical wound rotor with

split ring commutator.

Over to you:

1) Set up the series motor as shown above.

2) On your PC, run the program ’Open

Log_DC.bat’.

3) Run the program with the DC output set to

around 35%.

4) Make sure the motor is putting pressure on

the load cell rather than the balance - just

reverse the direction of rotation if it is not.

5) Press ‘Sweep’ to take an automatic set of

readings for a range of dynamometer loads.

6) Repeat steps 3, 4 and 5 for DC outputs of

40%, 45%, 50%, 55%.

7) When the sequence has finished load the

results into Excel and use Excel to plot a

graph of speed vs torque. 8)

8) On the next page sketch graphs to show

traces for all output voltages.

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Worksheet 9 Series wound motor characteristics

0 2500 Speed (rpm)

Torque(mNm)

0

80

40

1000 500 1500

20

60

100

2000

Over to you:

1) With the same equipment set up, load the program ’Open Control_DC.bat’.

2) Run the program and set the DC1 output to around 50%

3) Complete the table to show speed, output power, dynamometer power and efficiency.

The effect of speed on torque for different values of DC output voltage.

DC1 power

supply

Power

out

Dyno

power

Efficiency Speed

rpm

50%

60%

70%

90%

100%

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Worksheet 10 Universal motor

The series-wound motor is often called the

‘universal motor’ because it can be operated

from AC as well as DC.

This flexibility means that universal motors are

amongst the most widely used.

Typical applications include planes, routers,

sanders, grinders, saws etc.

This worksheet examines the AC behaviour of

the series-wound motor.

Over to you:

1) Set up the series motor as shown above.

2) On your PC, load ‘Open Log_1Phase.bat’ and

press ‘Play’.

3) With the output frequency set at 50Hz, run

the program and use the ‘Sweep’ function to

take an automatic set of readings over a

range of dynamometer loads.

7) When the sequence has finished, use Excel

to plot a graph of speed vs torque.

8) Repeat this procedure for frequencies of 60,

70, and 80Hz.

9) On the next page sketch graphs to show

traces for all frequencies.

Compare AC and DC properties of the universal

motor.

How do you change direction of motion?

Set the frequency to 40Hz - what happens?

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Worksheet 11 Universal motor

0 2500 Speed (rpm)

Torque(mNm)

0

80

40

1000 500 1500

20

60

100

2000

The effect of speed on torque for different values of output frequency.

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Worksheet 11 Comparing shunt and series motors

Over to you

1) Comparing the photographs, what is the difference between the rotor windings?

1)

2) Earlier you measured the resistance of the shunt and series motor windings.

Do these photographs corroborate your findings?

3) Research the term ‘separately excited’. What does this mean?

Shunt wound Series wound

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Worksheet 12 Single-phase induction motor

One problem with series and shunt motors is that

they use ‘brushes’ in the rotor circuit. These wear

out and need replacing from time to time.

Induction motors have no brushes and so require

less maintenance.

The photograph shows an induction motor and its

fitted gearbox.

Over to you:

1) Set up the single phase induction motor as

shown in the diagram above.

2) On your PC run ‘Open Control_1Phase.bat’

3) Connect the windings to terminals A and B.

When the software application is running,

these give a single phase output signal.

4) However, you will find that the motor does

not start.

Oh dear!

So what?

You have just discovered a problem with the

induction motor - it needs something to get it

started.

When the induction motor is powered, a little

physical nudge in one direction often starts it.

Unfortunately, in this rig, you cannot touch the

shaft, for safety reasons.

Another problem is that it does not matter

which direction you nudge the motor in - it can

rotate either way.

Next we will look at how to make the system

more predictable.

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Worksheet 13 Single phase induction motor - cap

start

In many situations, engineers have a choice over

which type of motors to use.

For simple AC applications, like pressure washers,

they can use either an induction motor or a series

motor.

The choice is governed by the electromechanical

properties of the motor - the cost, size and long-

term reliability required.

Over to you:

1) Set up the single phase induction motor as


2) On your PC load ‘Open Log_1phase.bat’ and

press ‘Play’.

3) With the output frequency set at 50Hz, run

the program and use the ‘Sweep’ function to

take an automatic set of readings over a

range of dynamometer loads.

4) When the sequence has finished, use Excel

to plot a graph of speed vs torque.

8) Repeat this procedure for frequencies of 60,

70, 80, 90 and 100Hz.

9) On the next page, sketch graphs to show

traces for all frequencies.

So what?

How do you change direction of rotation?

How do the speed / torque curves compare to

the other motors you have looked at?

What are the advantages and disadvantages of

DC and AC machines?

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Worksheet 13 Single phase induction motor - cap

start

0 2500 Speed (rpm)

Torque(mNm)

0

80

40

1000 500 1500

20

60

100

2000


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One of the drawbacks of the single-phase induction motor is

that it needs an external capacitor to make it operate.

The induction motor is very reliable because it has no brushes.

However the capacitor used to start the motor must be

switched out of the circuit once the motor is running.

This needs either additional electronic switching circuitry

or a centrifugal mechanical switch.

Over to you:

1) Set up the single-phase induction motor as

shown in the diagram. This experiment uses

the oscilloscope built into the control unit to

look at the phase shifts in the windings

caused by the Start/Run capacitors.

2) On your PC run ‘Open Control_1Phase.bat’.

3) Set the frequency to 50Hz.

4) Set the ‘scope to show I and IA, using the

‘Properties’ on the right hand side of the

screen. Capture the waveform.

5) Sketch what you see on oscilloscope grid on

the next page .

6) Repeat the procedure for frequencies of

60Hz, and 70Hz. For each frequency, record

the speed, currents and phase shift.

7) Investigate the effect of changing from the

‘Start’ to the ‘Run’ capacitor, using the

‘Capacitor Start / Run’ selector switch.

So what?

Without a capacitor, the motor would not

start. In some systems, the motor has two

capacitors, one value for starting and one

for running. The phase shift for each is

different.

For the same voltage, how much more efficient

is the ‘Run’ capacitor than the ‘Start’ capacitor

for each frequency?

Investigate why a different value is needed for

‘Start’ and ‘Run’.

Worksheet 14 Phase shift in induction motors

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Worksheet 14 Phase shift in induction motors

AC drive

frequency

Speed

rpm

I1 in A I in A Phase Efficiency

%

50Hz

60Hz

70Hz

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Worksheet 15 Variable frequency drive strategies

In factories, induction motors are often used at

fixed speeds, dictated by the 50 or 60Hz mains

supply.

Where speed control is needed it requires a

change in strategy.

This photograph shows variable frequency drives

used in a mineral water plant.

Over to you:

1) Set up the single-phase induction motor as


In this exercise, we are just looking at signals

and so the motor and dynamometer do not

need to be connected. However, the motor

must be in the dynamometer cradle in order

to activate the power supply . 1)

2) On your PC run ‘Open Control_1Phase.bat’. 3)

3) Connect the oscilloscope as shown. In this

application, the DC1 output gives a pulse at

the start of each cycle. This can be used to

trigger the scope. 4)

4) On the next page, sketch the waveform of

the single-phase AC output A.

So what?

This exercise illustrates how PWM (Pulse Width

Modulation) is used to create a waveform with

variable pulse width but constant period, the

period of the fundamental waveform.

The motor drive coils act as a low-pass filter for

AC signals, smoothing out the waveform. It

ends up looking much more like a traditional

sine wave.


Use a traditional ‘scope with no USB connection to the PC.

Modern electrical

machines



Over to you:

With the dynamometer load at around 50%, measure and record in the table below the speed,

torque, output power and input power, for frequencies of 50, 60 and 70Hz.

AC drive

frequency

Speed in

rpm

Torque Output

power

Input

power

50Hz

60Hz

70Hz

Modern electrical

machines



Over to you:

1) In the software application, click on the button marked ‘Sine’ . This changes the output of the drive

from pseudo-sine wave to a simple digital one.

2) Sketch the output at A when using the trigger pulse on DC1. (You could compare the signal from B.)

3) Then, with the dynamometer load at around 50%, measure and record in the table below the

speed, torque, output power and input power, for frequencies of 50, 60 and 70Hz.

So what?

It is surprising how well a simple digital drive signal works for the induction motor. However, when

energy conservation is paramount, the cost of a few bits of silicon needed to create the drive circuit

is of secondary importance.

AC drive

frequency

Speed in

rpm

Torque Output

power

Input

power

50Hz

60Hz

70Hz

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machines



Over to you:

1) In the block diagram of the control unit in the ‘Reference’ section, the motor drive outputs are

shown as ‘tri state’. This means that each output voltage can be high, low or open circuit.

The single phase 24V maximum outputs A and B are alternately positive and negative, effectively

creating a maximum peak-to-peak voltage of 48V across the motor.

2) Obtain the waveforms and sketch them on the axes below.

You can use the on board oscilloscope here to look at the current waveform.

However the on-board scope is not fast enough to show PWM accurately.

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Worksheet 16 Three-phase motor characteristics

This photograph shows a section of

a three phase motor with the outer

shell cut away. Note the lack of

brushes and the lack of a winding in

the rotor.

Induction motors work by inducing a

current, and magnetic field, into the

rotor which has a slightly different

phase to the driving current/field.

Over to you:

1) Set up the three phase induction motor as


2) On your PC, load ‘Open Log_3phase.bat’.

3) With an output frequency of 50Hz, run the

program and take an automatic set of read-

ings over a range of dynamometer loads.

4) Repeat this for output frequencies of 60, 70,

80, 90, 100 Hz.

5) If you can plot all output lines on the same

graph.

6) On the next page, sketch the results .

How do you change direction of rotation?

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Worksheet 16 Three phase motor characteristics

0 2500 Speed (rpm)

Torque(mNm)

0

200

100

1000 500 1500

50

150

250

2000

So what?

Compare the speed/torque curves to those of the single-phase motor.

How does it compare for efficiency? Research the reason for this and give your findings below.


Modern electrical

machines


Worksheet 17 Three-phase motor drive strategies

Over to you:

1) Set up the three-phase induction motor as

shown in the diagram above. As this exercise

looks only at the signals the generator unit

creates, there is no need to connect the

three-phase motor to the supply.

2) Connect three 10k resistors as loads

between U, V, W and 0V.


The National Grid distributes electricity in three separate

phases rather than the single-phase found in homes.

Three-phase generation is more energy efficient.

Factories use three-phase motors as they too are more

efficient than single-phase systems.

The photograph shows a three-phase induction motor

factory in China.

4) Press ‘Play’ and put the software into digital

mode by pressing the ‘Sine’ button. This

changes the waveform from pseudo-sine,

constructed with PWM to a simple digital

waveform where U, V, and W are 120 degree

shifted square waves. Set the output fre-

quency to 50Hz and press ‘Run’.

5) Trigger an oscilloscope on the DC1 output.

On the second channel display the output U.

6) Sketch the waveforms of signals U, V, and W ,

relative to the timing waveform on DC1.

Modern electrical

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Worksheet 17 Three phase motor drive strategies

So what?

Notice that these pulses are 120o apart. Together, they make up the basic three-phase waveform.

You should notice that each waveform spends some time at 0V, some time at 24V and some time

when it is in between, at around a volt or so. What you are seeing is the three stages of drive of each

output - forced to 0V, forced to +24V and left open circuit. In this last stage, the voltage is not dictated

by the power supply but by the rest of the circuit.

Modern electrical

machines


Worksheet 17 Three phase motor drive strategies

Over to you:

1) Still with the 10k resistive loads in place, change the drive waveform to pseudo sine at 50Hz.

4) Once again, sketch the waveforms of signals U, V, and W , relative to the timing waveform on DC1.

So what?

You now see the waveforms that make up the three-phase drive. There are in fact six separate steps

to this. Hence it is often called the ‘six step three-phase motor drive system’.

Some three-phase speed control systems use simple square waves for driving motors. However the

pseudo-sine wave you have examines is much more efficient.

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Worksheet 18 Understanding slip

Whilst induction motors may vary in size, their

characteristics are all similar.

Larger motors can be very expensive. When

they fail, they are refurbished by hand - a

specialist task.

In the photograph, an engineer is re-winding

an induction motor.

Over to you:

1) Set up the three-phase induction motor as



3) Calculate the synchronous speed for the

three-phase motor using the formula in the

‘Key relationships’ section.

(Refer to the cross-section drawings in the

‘Reference’ section to see how many poles

this motor has.)

4) With no dynamometer load, for drive

frequencies of 40Hz, 50Hz, 60Hz, 70Hz and

80Hz, calculate the slip at each frequency.

Record the results in the table opposite.

5) Set the drive frequency to 50Hz. Increase the

dynamometer load to 50%, then 75%, then

90% and finally full load. Calculate the slip at

each step. Record the results in the table.

3p motor

drive

Speed Slip

40Hz

50Hz

60Hz

70Hz

80Hz

Dyno

load

Speed Slip

0%

50%

75%

90%

full

Slip vs drive frequency - no load

Slip vs load - drive frequency = 50%

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machines


Worksheet 18 Understanding slip

Over to you:

Explain these readings.

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machines


Brushless DC motors have a great power

output to weight ratio which makes them

useful in weight sensitive applications like

drones.

Their construction means that they need

to be started at a slow speed and then

ramped up to full speed.

Over to you:

1) Set up the brushless motor, as shown in the

diagram above. Don’t forget to connect the

brushless DC motor to the control unit with

the 5-pin lead.

2) On your PC load ‘Open Log_3phase.bat.’

3) With the initial frequency set to 40Hz, run

the program to get a set of readings.

4) Repeat this step for drive frequencies of 50,

60, 70, and 80Hz.

5) On the next page, sketch the results.

Worksheet 19 Brushless DC motor

Find out why a brushless DC motor is powered

by a three-phase AC supply.

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Worksheet 19 Brushless DC motor

0 2500 Speed (rpm)

Torque(mNm)

0

80

40

1000 500 1500

20

60

100

2000


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We see small wind turbines everywhere.

The efficiency of three-phase over single-phase

means that even small wind generators use

three-phase machines to generate electricity.

In this experiment, the brushless DC motor acts

as a three-phase generator. It is mechanically

linked to, and driven by, the dynamometer,

which is supplied with a DC voltage.

Over to you:

1) Set up the brushless motor as shown in the

diagram above.

2) On your PC load ‘Open Control_DC.bat’.

3) Set a duty cycle of 40% on the DC1 output.

4) Press the ‘RUN’ button on the software.

5) Vary the DC output from 40% to 100% in 10%

steps.

6) Make a note of the peak to peak output volt-

age from the brushless motor and fill in the

table on the next page.

7) Sketch the three phase waveform on the next

page.

8) How does the waveform relate to the motor

RPM?

So what?

The great thing about the brushless motor is

the small size of the package and the fact that

there are no brushes for ease of maintenance.

Worksheet 20 Three-phase generator


3p motor

drive

Speed

rpm

Vout

pk-to-pk

40%

50%

60%

70%

80%

90%

100%

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machines


Worksheet 20 Three phase generator

Over to you:

Use the tools available to investigate the efficiency of the brushless motor and compare it to that of

the DC motor.

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machines


Machine information

Reference

Modern electrical

machines


Reference The dynamometer

Conventional symbol

Actual circuit

Circuit in use

Construction

Stator winding circuit

Connection Colour

V+ Red

V- Black

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machines


Reference The dynamometer

Modern electrical

machines


Conventional

symbol

Actual circuit

Circuit in use

Construction


Connection Colour

Armature Purple

Armature Yellow

Field coil Black

Field coil Red

Reference The shunt motor

Modern electrical

machines


Reference The shunt motor

Modern electrical

machines


Construction


Reference The series motor

Conventional

symbol

Actual circuit

Circuit in use

Connection Colour

Armature Purple

Armature Yellow

Field coil Black

Field coil Red

Modern electrical

machines


Reference The series motor

Modern electrical

machines


Reference The DC motor

Conventional symbol

Actual circuit

Circuit in use

Construction


Connection Colour

V+ Red

V- Black

Modern electrical

machines


Reference The DC motor

Modern electrical

machines


Reference The single phase AC induction motor

Conventional symbol

Actual circuit

Circuit in use

Construction


Connection Colour

Run Grey

Start Brown

Common Blue

Modern electrical

machines


Reference The single phase AC induction motor

Modern electrical

machines


Reference The three phase AC induction motor

Conventional symbol

Actual circuit

Circuit

Construction


Connection Colour

Phase U1 Black

Phase U2 Blue

Phase V1 Grey

Phase V2 Blue

Phase W1 Brown

Phase W2 Blue

Modern electrical

machines


Reference The three phase AC induction motor

Modern electrical

machines


Reference The Brushless DC motor

Construction


Connection Colour

Phase U Black

Phase V Grey

Phase W Brown

Conventional

symbol

Actual circuit

Circuit in use

Modern electrical

machines


Reference The Brushless DC motor

Modern electrical

machines


Package information

Reference

Modern electrical

machines


Reference

Understanding the system

1

2 4 3

5 6

7

8

9

The system consists of a number of 24V electri-

cal machines, a control unit, software applica-

tions for driving the control unit and a set of

worksheets. The photograph above shows

these parts. They are:

) The dynamometer and cradle which con-

nects to the control unit using a 25way D-

type lead

) Balance

) Motor under test - in this case a shunt

wound motor

) The control unit which connects to a PC

using a USB lead.

) A series wound motor

) Single-phase AC induction motor

) Brushless DC motor

) DC motor

) Three-phase AC induction motor

) The software application running on a PC

10

Modern electrical

machines


Reference

Understanding the control unit

) The dynamometer resistance: use this to

control the effective resistance placed

across the dynamometer: low electrical

resistance means a large mechanical re-

sistance, and high electrical resistance

means low mechanical resistance

) The dynamometer connections

) The speed of the motor in Revolutions Per

Minute - RPM.

) The power LED which shows the control

unit is powered up.

) The COMMS LED which is lit when the PC

software has communication with the

control unit.

) The ‘Motor In’ LED which indicates that a

machine is physically connected to the

dynamometer.

) The internal ammeter and voltmeter con-

nections

) The DC 1 Supply output control.

) The DC 1 connections.

) The DC 2 Supply output control.

) The DC2 connections.

) The AC frequency control

) The three-phase supply connections

) The single phase supply connections

) The capacitor mode selector switch: this

controls the internal value of capacitor

connected to the A and B terminals. There

are two values: START, RUN.

) Variable capacitance terminals.

Note - manual controls are overridden as soon as the unit is plugged into a computer.

1

2

4

3

5

6

7

8 10

9 11

12

13

14

15

16

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) The Mains input plug - the unit takes 240V

or 110V

) The USB connector

) A 25 way D-type connector which is used

to connect the Control unit to the Dyna-

mometer

) Vent holes for internal fan - do not cover.

1

2

3

4

Reference

Understanding the control unit

Modern electrical

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Reference

Control unit schematic

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machines


Reference

Control unit description

Please refer to the block diagram on the previ-

ous page.

The control unit shipped with the Matrix Mod-

ern Electrical Machines system is one of the

most up to date in the World. Inside the unit

there are three separate processors: a main

processor handling all user interface and com-

munication tasks, a motor drive processor han-

dling all the high current outputs and dealing

with all waveform and timing systems, and a

measurement processor. These three proces-

sors use buses to communicate between

themselves and between the different elec-

tronic circuits in the unit.

A key feature of the control unit is that almost

every quantity in the unit can be measured. On

the schematic you can see that there are

around 13 separate ammeters and voltmeters

in the system. This gives the instrumentation

software and the user lots of options when dis-

playing what is happening in an electrical ma-

chine system. The measurement of each quan-

tity takes place thousands of times per second

and can be processed by PC software applica-

tions to display a quantity or a waveform.

The motor drive chips are all low voltage drop

FET based units. As you can see from the sche-

matic all the outputs are digital 24V outputs.

Simple pulse width modulation algorithms are

used to vary the effective output power on the

DC outputs. On the AC outputs Pulse width

modulation is again used but with a more ad-

vanced pseudo-sine wave algorithm which var-

ies the output power sinusoidally over the pe-

riod of the output waveform. This technique is

used by the more advanced motor controllers

in industry. Older, and perhaps cheaper, motor

driver systems sometimes use a simple digital

output waveform for driving three phase in-

duction motors where the three outputs are

simply digital outputs phase shifted by 120 de-

grees. This digital output is also available in

some software applications shipped with the

system so that students can investigate the

efficiency of each method.

The motor driver chip outputs can be placed in

one of three states: 24V, 0V and open circuit.

This allows for different drive strategies when

generating waveforms for driving motors. As

part of the learning package students investi-

gate PWM for driving DC motors, and Pseudo-

sine waves for driving single phase AC motors.

The unit also performs more complex six step

pseudo-sine wave generation for driving the

three phase motor and this can be examined

with the internal multi channel oscilloscope as

well as an external oscilloscope. To facilitate

this when in AC mode a small pulse is given on

the DC1 output which allows an external oscil-

loscope to be triggered to capture the various

waveforms.

The unit includes two values of capacitor with

a simple relay for start / run investigation for

single phase induction motors.

The unit also includes a FET based load for the

dynamometer whose effective resistance is

controlled by software.

Modern electrical

machines


Reference

Understanding the dynamometer

A dynamometer is an instrument that directly

measures the force produced and speed of a

rotating machine (the motive force). Torque

and power can be calculated from the meas-

ured force and speed. In our case the mechan-

ical rotation is produced by one of the DC or

AC electrical motors. This Dynamometer is an

absorption type swing arm / balanced beam

dynamometer, where the motive force is used

to drive a DC generator with a variable re-

sistance load attached. The resulting force can

be measured in one of two ways:

• A digital scale (shown in the image above)

can be used to measure the effective ver-

tical force on the scale and it gives a read-

ing in Kilograms. This can be converted

into newtons and newton metres - see

below.

• A load cell (behind the dynamometer and

not shown in the image above) connected

to the control box gives the same infor-

mation - give or take a few percent.

Note that the rotation of the dynamometer

dictates whether the balance or the load cell is

used. Controlling the direction of motion of

each machine is discussed below.

The encoder on the shaft of the dynamometer

allows the control unit to detect the rotational

speed. This is displayed on the control unit in

revolutions per minute (rpm) and on the soft-

ware application.

Machines under test are coupled to the dyna-

mometer using a coupling which is housed in a

plastic tube to prevent clothes and hair getting

caught in the mechanism. Two microswitches

are provided on the dynamometer - unless a

machine under test is in place the control unit

will not activate.

Safety switch

DC generator

Digital scales

Machine under

test

En

co

der

Dynamometer

Swinging arm

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machines


Reference

Measuring torque

The dynamometer rotates in the same direc-

tion as the machine under test. It provides a

mechanical resistance for the machine under

test. It also generates a voltage and - if a re-

sistance is placed across the Dynamometer ter-

minals - it produces a current.

If voltage and current flow then power is being

generated - work is being done. The more

power generated by the Dynamometer, the

more mechanical power has to be generated

by the machine under test.

We test electrical machines to understand

their properties so we know when to use the

various types of machine. The way we test

them is to vary their speed and the mechanical

power they need to generate. The dynamome-

ter and the Control unit allow us to do this.

The Dynamometer is made up of a DC motor

and a cradle. The DC motor also acts as a DC

generator. The cradle suspends the DC ma-

chine in two bearings that allow the body of

the machine to rotate. The force on the load

cell (or digital scale when the motor is running

anticlockwise) creates a weight on the load cell

or scale (which represents mass). If the swing

arm is perfectly balanced and horizontal when

it is at rest, the force produced on the load cell

or digital scale is Force = mass x 9.81m/s2. The

torque can be calculated from the formula

Torque = Force x Distance. In this case, the dis-

tance is from the centre of rotation of the gen-

erator to the centre line of action of the swing

arm. For the Matrix dynamometer, r =

38.12mm

More discussion on calculations is given below.

Motor must run clockwise for

load cell mode

Motor must run anti-clockwise

for digital scale mode

r

F=mg

Torque = mgr

Scales

Swinging arm

Modern electrical

machines


Software

Information

Reference

Modern electrical

machines


Reference

Equipment: modes of use

The Modern Electrical Machines equipment

can be used in two ways:

Manual mode - no software

When no USB lead is plugged into the unit then

the control unit is in manual mode. This allows

users control of the DC and AC power supply

outputs using the potentiometers on the front

of the unit. The capacitor start / run is con-

trolled by the front panel switch and the speed

of the dynamometer is displayed by the 7 seg-

ment display. Manual mode is useful when

teaching students the basics of electrical ma-

chine control and will be useful when used

with true RMS multimeters to measure voltag-

es and currents.

In Manual mode we make use of a small bal-

ance to measure torque. The swinging arm of

the Dynamometer has two modes of use:

when rotating counter-clockwise it presses

down on the balance. When rotating clockwise

it presses down on the load cell for computer

based measurement. Users simply reverse the

direction of motion of the machine under test

to change rotational direction.

Software mode

When the USB lead is plugged in to the Control

unit the front panel controls are disabled. Six

separate software applications are available to

the user for carrying out experiments with the

different machines.

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machines


Reference

Understanding the software

Applications

Six different PC based applications are shipped

with the unit. Each of them is a self contained

executable ‘*.BAT’ file. Drivers for the Control

unit will need installing - the applications will

not. These applications are:

Open Control_DC.bat: for direct DC measure-

ment for DC motors.

Open Log_DC.bat: for gathering speed torque

data and logging the results for DC motors.

Open Control_1Phase.bat: for direct measure-

ment for single phase AC induction machines.

Open Log_1Phase.bat: for gathering speed

torque data and logging the results for single

phase AC induction machines.

Open Control_3Phase.bat: for direct measure-

ment for three phase induction and brushless

DC motors.

Open Log_3Phase.bat: for gathering speed

torque data and logging the results for three

phase induction and brushless DC motors.

These are explained below.

An API for use with LabView or MATLAB is also

available.

Modern electrical

machines


Reference

Software flags

Each application has a set of software flags

which are displayed on the top left hand side.

These are:

Application: green when the application is in

Play mode.

USB: Green when there is a USB connection

between the control unit and the PC.

Error: red when there is an Error in the control

unit. Most often this happens due to an over

current error. The unit is capable of providing

100 watts of power. When this is exceeded the

unit shuts down. In this case you should lower

the output voltage and try again.

Motor: green when the microswitches in the

Dynamometer are activated by the presence of

a machine under test. The unit will not run un-

less a machine under test is present.

Direction: green when the direction of rotation

is clockwise which will press down on the

torque sensor.

Run: green when the run switch is triggered.

Modern electrical

machines


There are two types of software application

supplied with the Electrical Machines course:

• Manual control

• Automatic

Manual control applications allow you to man-

ually set all of the parameters inside the con-

trol box. Automatic applications allow you to

automatically take multiple sets of readings for

producing speed torque graphs. This page and

the following 5 pages show you the screen lay-

outs and controls.

The file name for this program is ‘Open Con-

trol_DC.bat’.

Manual DC measurement

1) Program control - hit play to start

2) Flags for App running, USB comms and Motor

in place etc. The box will not start until all

show green ticks

3) DC1 output - shows PWM duty cycle for DC

output 1 with voltage and current.

4) Run switch - needs pressing to turn outputs

on


output 2

6) Meter for Current terminals

7) Meter for Voltage terminals

8) Dynamometer load as a %

9) Dynamometer speed in RPM

10) Torque in mNm

11) Dynamometer power in watts

12) Oscilloscope

13) Properties for scope.

1

2

4 3

5

6

7

8

10 9

11

12

13

Reference

Understanding the software

Modern electrical

machines


The file name for this program is ‘Open

Log_DC.bat’



in place. The box will not start until all three

show green ticks


output 1

4) Sweep - takes a set of readings sweeping the

Dynamometer from 0-% to 100%


on


output 2

7) Meters for DC 1 - RMS values

8) Meters for DC 2 - RMS values


10)Dynamometer speed in RPM

11) Torque in mNm


13) Internal ammeter and voltmeter V1 and I1

14) On screen graph of plot data

15)Graph / log clear button

16) Properties

When the Run button is pressed the control box

will - at the specified voltage on DC1 and DC2

take readings from 0 load to 100% load in 2%

increments. The results will be stored in the file

LOG_DC1.CSV for plotting in Excel.

Whilst the readings are being taken the on

screen graph will show an approximate plot so

that you can be sure you are getting valid data.

Up to 8 data sets can be captured and displayed

at one time.

Reference

Automatic DC software

1

2 4

3 5

6 7 8

10

9

11

12

13 14 15

16

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Reference

Single phase AC manual


trol_1Phase

.bat’




show green ticks

3) Frequency of output




on

6) Sine/Digital - default is pseudo-sine, on is

trapezoid.

7) Start run switch controls the value of internal

capacitor on the variable capacitor terminals

8) Meters for the outputs - RMS values


10)Dynamometer speed in RPM

11) Torque in mNm



14) Oscilloscope plot area

15) Oscilloscope trigger switch

16) Properties

1

2 4

3 5

6

7

8

10

9

11

12

13

15 14

16

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Reference

Single phase AC automatic


Log_1Phase.bat’




show green ticks


4) Run switch - starts automatic logging of data

5) Drivemode - default is pseudo-sine, on is

trapezoid

6) Start run switch controls the value of internal

capacitor on the variable capacitor terminals

7) Meters for the output - RMS values



10)Torque in mNm (Zero switch is used on start

up)

11)Dynamometer power in watts

12)Internal ammeter and voltmeter V1 and I1


14)Graph / log clear button

15) Properties


will - at the specified frequency output take

readings from 0 load to 100% load in 2% incre-

ments.

The results will be stored in the file

LOG_1phase.CSV for plotting in Excel.




The Start Run switch is handled automatically.

1

2 4

3 5

6

7

8

10 9

11

12 13

15

14

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Reference

Three phase AC manual


trol_3Phase.bat’



in place.





on

6) Sine / Digital - default is pseudo-sine, on is

trapezoid




10)Torque in mNm


12)Internal ammeter and voltmeter V1 and I1

13) Oscilloscope plot area

14) Oscilloscope trigger switch

15) Properties

This application runs the three phase motor and

the Brushless DC motor.

1

2

4

3 5

6

7

8

10 9

11

12 13

15

14

Modern electrical

machines


Reference

Three phase AC automatic


Log_3Phase.bat’



in place.


4) Run switch - needs pressing to start the

sweep

5) Drivemode - default is pseudo-sine, on is

trapezoid




9) Torque in mNm




13) Graph / log clear button

14) Properties

This application runs the three phase motor and

the Brushless DC motor.


will - at the specified frequency output take

readings from 0 load to 100% load in 2% incre-

ments.

The results will be stored in the file

LOG_3phase.CSV for plotting in Excel.




Up to 8 data sets can be captured and displayed

at one time.

1

2 4

3

5 6

7

8

10

9

11 12

13

14

14

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machines


Reference

The on- board oscilloscope

The oscilloscope in the Control unit takes

measurements of every ammeter and voltme-

ter in the unit at a rate of 64kHz. There are 16

channels in total - that gives us an effective

measurement rate of 4kHz for each channel.

Y axis, Timebase and traces can be set by click-

ing on the appropriate words in the Properties

section at the right hand side of each applica-

tion.

To alter the time base: click on the oscilloscope

area and use the mouse wheel to zoom in and

out.

The oscilloscope is useful for seeing voltages

and currents at close to the driving frequen-

cies. In fact we use a small capacitor on the

input to deliberately slow the effective rate of

sampling to make the instrument work better

for this purpose.

Because the PWM frequency used is 7kHz and

the effective sampling rate is 4kHz then

Nyquist effects can be seen when zooming in

to examine the PWM drive frequency. Because

of this an external oscilloscope needs to be

used when examining waveform structure in

detail.

The oscilloscope is particularly good at showing

currents the pattern of voltage and current

and the phases between voltage and current.

Modern electrical

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Reference

Software installation

Copying over the software

The software for Electrical Machines is available as a

download from the Matrix web site. When you un-

pack the zip file you will see the following files:

There are 6 applications that are used for the Electri-

cal Machines range. These are documented below.

Please copy these programs and directories into a

suitable location on your hard drive. You can make

short cuts to the *.BAT files if you wish.

Rin each program by clicking on the ‘*.BAT’ file.

Installing drivers

Go into the USB drivers folder and run the 32 bit or

64 bit as appropriate for y our computer. This should

install the driver for the control box.

You can check the driver is installed properly by look-

ing at Device Manager and checking that the Electri-

cal Machines controller appears under Ports - see op-

posite.

Modern electrical

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Instructor Guide

Modern electrical

machines


About this course

Introduction

The course is essentially a practical one. Where possible, practical implications and applications of the theory

are highlighted to make the course more relevant to the students.

The equipment offers tremendous scope for additional research on the topic and instructors may wish to

encourage and support this research.

Aim

The course introduces students to concepts and devices used in a range of electrical machines. It covers much

of the content of Unit 15 - Electrical Machines in the BTEC Level 3 Diploma in Electrical and Electronic Engineer-

ing, and similar qualifications.

Other modules relevant to the Diploma course are:

LK9392 - ‘Introduction to digital electronics’; LK8473 - ‘Advanced electrical principles - DC’;

LK8749 - ‘Advanced electrical principles - AC’; LK2686 - ‘Three-phase systems’;

LK3568 - ‘Power and Energy Electronics’.

Prior Knowledge

It is recommended that students have followed the ‘Electricity Matters 1’, ‘Electricity Matters 2’ courses, or

have equivalent knowledge, covering the basic electrical concepts of current, voltage and resistance, energy

and power and are able to construct and test circuits, using a multimeters and oscilloscopes.

About these worksheets

These worksheets provide a practical environment for the study of Modern Electrical Machines. This is not the

only resource a student will need. Students will also benefit from lectures, tutorials, text books and other

provided by the instructor. The experiments in this course should be integrated with teaching to introduce the

theory behind it, and reinforced with written examples, assignments and calculations. The internet is a rich

source of information on this topic and the instructor should give students research assignments accordingly.

We have set out the learning resource as a series worksheets, followed by a Reference section. It is expected

that the students will be given their own copy of the worksheets, either one by one, or as a pack.

This format encourages self-study, with students working at a rate that suits their ability. The instructor should

monitor that students’ understanding keeps pace with their progress through the worksheets. One way to do

so is to ‘sign off’ each worksheet, as a student completes it, and in doing so have a brief chat with the student

to assess grasp of the ideas involved in the exercises it contains.

‘Answers’ are not included. They are easy to calculate and are often subjective.

Time:

It should take students between 12 and 17 hours to complete the worksheets. (A similar length of time will be

needed to support the learning that takes place as a result.)

Instructor Guide

Modern electrical

machines


Learning Objectives

On successful completion of this course the student will be able to:

• describe the precautions required when operating electric motors;

• calculate the force exerted by an object due to gravity, given its mass and the acceleration due to gravity;

• explain what is meant by the term ‘torque’;

• calculate the torque about an axis, given the force and distance from the axis

• explain the meaning of the term ‘power’ and calculate the power delivered by:

• a rotating shaft;

• a DC electric motor;

• a single-phase AC electric motor;

• a three-phase electric motor;

• explain the meaning of the term ‘energy efficiency’ and calculate it for an electric motor;

• explain the meaning of the term ‘synchronous speed’ and calculate it for an AC motor;

• explain the meaning of the term ‘slip’ and calculate it for an AC motor;

• describe the structure of a simple DC motor;

• describe the function of a commutator in a DC motor;

• take measurements that allow the torque applied by a rotating motor to be measured;

• describe how to reverse the direction of rotation of a DC motor;

• describe what is meant by ‘pulse-width modulation, (PWM)’ and explain its use in controlling the speed of a

DC motor.

• explain what is meant by the term ‘mark:space’ used in connection with a PWM signal;

• describe the energy flow when a powered DC motor is connected to a dynamometer which has a resistive

load attached to it;

• describe three domestic or industrial applications of the DC motor;

• describe the effect of rotational speed on torque for a DC motor;

• take measurements to investigate the variation of efficiency with speed for a dynamo under load;

• describe the differences between a shunt-wound and series-wound DC motor;

• explain the meaning and significance of the term ‘back e.m.f.’;

• measure the resistance of the windings in series-wound and shunt-wound DC motors and explain the signifi-

cance of the results;

• describe how to reverse the direction of rotation of a wound-stator DC motor;

• explain the meaning of the term ‘separately excited’ when applied to wound-stator DC motors;

• compare the performance characteristics of shunt-wound and series-wound DC motors;

• explain why series-wound motors are widely used and list six common applications;

• describe the main difference in structure between a universal AC motor and a single-phase AC

induction motor;

• explain the need for a starting circuit in a single-phase AC induction motor;

continued on next page ...

Instructor Guide

Modern electrical

machines


continued ...

• describe how to reverse the direction of rotation of an AC induction motor;

• explain the need for ‘start’ and ‘run’ capacitors in a single-phase AC induction motor;

• draw a graph to illustrate the phase shift between currents flowing in the windings of an AC induction motor;

• describe the use of a variable frequency drive for speed control in an AC induction motor;

• explain what is meant by a ‘tri-state’ driver;

• distinguish between a single-phase and a three-phase AC supply;

• describe how to reverse the direction of rotation of a three-phase AC motor;

• compare the performance of a single-phase and a three-phase AC induction motor;

• investigate the variation of slip with speed and load for a three-phase AC induction motor;

• describe the significant features of a ‘brushless’ DC motor;

• describe the power supply requirements of a brushless DC motor;

• describe the speed/torque characteristics of a brushless DC motor;

• investigate the use of a brushless DC motor as a three-phase generator;

• investigate how the efficiency of the brushless motor compares to that of the DC motor.

Instructor Guide

Modern electrical

machines


Worksheet Notes for the Instructor Time

1

Getting start-ed

The first worksheet looks at setting up the system. It is important that students take care to line up the components as directed in the worksheet. The instructor should check that the balance has been positioned correctly.

The balance is used when in manual mode to obtain a mass (and hence force) reading. The software receives torque data from the load cell.

Instruction (3), about software applications and drivers may not be relevant to students in some establishments but it is given as a reminder to thoe responsible for such matters.

This is a good time to go over the health and safety aspects of using the kit.

The students should be encouraged to read the ‘Key Relationships’ section on page 4 and to familiarise themselves with the content of the extensive ‘Reference’ section, particularly page 63, which allows them to identify the various components in the kit.

20 - 30 mins

2 Understanding

torque and mass

Here the students learn how to use the dynamometer to measure torque. The kit is used in manual mode and students should read carefully the description of the dynamometer given on page 68 before attempting to use the kit.

Torque can be derived using the balance reading and applying the appropriate formula in the ‘Key Relationships’ section. The Instructor may need to reinforce ideas about the concepts involved - mass, force and torque in particular.

Students may need help with some of the calculations depending on previous experience and mathematical ability.

40 - 60 mins

3 Understanding the DC power

supply

The focus here is on pulse-width modulation and its use to control motor speed. An introduction to the topic by the instructor will ease the students’ progress through the exercise. They need to understand the concept of mark:space ratio.

The worksheet also allows them to practice triggering a ‘scope. Once again, an introduction by the instructor will help where students have not met the tech-nique before.

A key technique in the learning resource is the sketching of waveforms. Students need to get used to doing this, not in a highly accurate way, but in a way that conveys key timing information.

40 - 60 mins

4 DC motor

characteristics - 1

This worksheet introduces the operation of probably the most widely used low-power motor, the DC permanent magnet motor. With the benefit of modern rare earth magnets, this type of motor is a viable alternative to traditional wound sta-tor motors. The lack of stator windings means that this type of motor can be smaller than its counterparts. This motor produces more torque than any of the other motors in the Matrix set.

In this exercise, students use a software application to examine the motor’s per-formance. Students should appreciate that they are not seeing an on-screen sim-ulation, but are using SCADA (supervisory control and data acquisition,) to collect data in real time from the motor. The students sketch a graph based on readings from the computer. This recognises that there is a danger, when using IT, that students press buttons rather than understand. The idea of sketching some of the graphs is to improve understanding and retention.

The instructor may wish to focus on the meaning of power and efficiency.

50 - 70 mins

Instructor Guide

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5 DC motor

characteristics - 2

In this worksheet students use a logging application to capture the characteristics of a DC motor for a range of output voltages. It is suggested that students then view that data in ‘Excel’ using that application’s graph-plotting function.

Using Excel in this way is a useful key skill. What we want to avoid is students just copying and pasting graphs into a Word document without understanding their significance. Using ‘Excel’ for all the worksheets is probably too time consuming and so the instructor needs to use judgement here.

A key issue here is the equivalent data for the load that the motor will be driving and how it is plotted on the same graph. Understanding the load / speed / torque requirements for an installation is not examined in the worksheet but is a valid extension at this point should the instructor wish it.

50 - 70 mins

6 Permanent

magnet generator (dynamo)

It is important that by this stage, students have a firm grasp of the meaning and significance of the concepts power and efficiency. Here, the motor delivers mechanical power to the dynamo contained in the dynamometer. In turn, it delivers electrical power to the load. Taking appropriate readings, the students calculate the energy efficiency of this conversion for a range of supply voltages (and hence speeds.) The results are plotted as a graph.

Should the instructor think it appropriate, students could use ‘Excel’ to produce the graph.

30 - 50 mins

7 Shunt and

series motor basics

This exercise focuses on the construction of motors and the significance of the terms ’shunt’ and ’series’. The photographs in the ‘Reference’ section are a valu-able aid. Students should study them carefully and could be asked to identify the salient features of each.

First of all, they test the windings of motors, using an ohmmeter. Depending on previous experience, the students may require input from the instructor to help them with this.

Students also need to understand the concept of ‘back emf’. They measure the absolute resistance of the windings and might expect to use this to predict the current flowing in them. However, when the winding is in the motor, rotating inside a magnetic field, a voltage (back emf) is induced which opposes the driving voltage, reducing the overall voltage experienced by the motor. As a result, a smaller current flows. It is a concept that will take time to develop. At this stage, students are not expected to perform calculations on it.

The second part of the worksheet investigates the behaviour of the two types of motor, when running. The circuits are different - two separate power supplies are used for the shunt motor, but only one for the series motor. For some stu-dents, it is advisable to check the wiring. Students measure the current through the coils and rotational speed over a range of supply voltages.

The series motor has an extremely small resistance, meaning that huge currents can flow. The control unit will cut out eventually, when the series motor is used. This is a safety precaution to prevent the control unit from overheating, and is a feature of all well-designed power supplies.

30 - 50 mins

Instructor Guide

Modern electrical

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8 Shunt-wound

motor characteristics

This worksheet investigates the operation of the separately-excited DC shunt motor to obtain the speed/torque curve for the machine.

A key aim of the module is to get student to understand that electrical machines have different characteristics. Different applications have different demands and the engineer will be tasked with choosing a motor with characteristics appropri-ate to meeting those demands. Understanding speed, torque and efficiency and their roles in motor operation are a big part of this.

As pointed out in the last worksheet, the shunt motor uses separate supplies for the armature (or rotor) and the field windings (stator.) For some students, it is advisable to check the wiring.

The software produces lovely speed/torque curves. The reason for asking students to redraw them by hand is to encourage them to think a little more about the numbers!

The activity ends with an investigation into the effect of the field windings voltage on the energy efficiency of the motor. There is scope with this arrangement to expand the investigation into factors affecting efficiency

40 - 60 mins

9 Series-wound


Next, students investigate the series-wound motor in the same way.

The DC power supply is designed to ramp up to the required value when you ‘Run’ the application, rather than deliver the final value straight away. Part of the reason for this is to limit the current on start up. With more able students, the instructor could lead a discussion on back emf and its implications for series and shunt motors.

Typical of all wound-rotor motors, to reverse the direction of rotation requires reversing the polarity of one (only) of the windings. Reversing all power supply connections keeps the motor rotating in the original direction since both fields have changed polarity.

Again, for the reason given in the last worksheet, students sketch graphs by hand, as well as obtaining them from ‘Excel’.

Mirroring the investigation into the shunt motor, the activity ends with the effect of the power supply voltage on the energy efficiency of the motor.

40 - 60 mins

10 Universal

motor

This worksheet builds on the knowledge gained in worksheet 9, using the same series-wound motor to investigate the operation of the universal motor, now powered by AC.

This motor performs better (i.e. produces higher torque) when running as a DC motor. When running as an AC motor, the increase in impedance due to the AC causes a reduction in the motor current. As with all motors, lower current means lower torque. The instructor could discuss the effect of frequency on impedance as part of the introduction to this investigation.

Afterwards, students are asked to compare the characteristics of this motor in its DC and AC modes of operation.

40 - 60 mins

Instructor Guide

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11 Comparing shunt and

series motors

Students compare the construction of the two types of motor, using the photo-graphs provided. They should study closely the photographs on pages 50 to 53 of the ‘Reference’ section, as well. (Please discourage them from taking the actual machines apart!)

The instructor should remind them of their earlier findings on the resistance of the windings and current demand of the two types of motor.

15 - 25 mins

12 Single-phase

induction motor

This worksheet introduces the AC single-phase induction motor. The name ‘induction’ refers to the way in which the current is induced in the rotor by the moving magnetic field created by the AC current in the field windings.

Students find out that this type of motor does not self-start. They could be given the task of researching various starting / running circuits used with the induction motor. The instructor could discuss the meaning and cause of phase shift at this point, in anticipation of the coming investigations.

The induction motor usually has two separate coils in the field windings, a ‘main’, or ‘run’ coil and an ‘auxiliary’ or ‘start’ coil. The ‘start’ and ‘run’ windings of this motor are identical, so they windings can be swapped in order to change the direction of rotation of the motor. One end of the ’run’ winding is connected to one end of the ’start’ winding to form the ‘common’ connection.

To run the motor in the opposite direction, stop the motor and reverse the con-nections to the dynamometer and the ‘nudge’ will cause the motor to reverse.

15 - 25 mins

13 Single-phase

induction motor - cap

start

In this exercise, a capacitor is incorporated to create a phase shift between the magnetic field of the ‘run’ coil and that of the ‘start’ coil. We now have a self-starting induction motor.

The phase shift is frequency dependent. This is be one outcome of interpreting the graphs resulting from the measurements. Ask the students to remove one of the windings and observe that the motor still rotates – once started.

40 - 50 mins

14 Phase shift in induction

motors

This worksheet reinforces the notion of a phase shift caused by the start / run capacitors and the resulting shift in the magnetic fields inside the motor. It ex-plores the link between AC driving frequency, speed and energy efficiency.

The internal oscilloscope has been added to the control unit for this purpose and you will find that will a little practice it is a superb tool for understanding phases involved in driving the motor. The instructor may be in demand to help with using the software, controlling the oscilloscope and interpreting the results.

40 - 50 mins

15 Variable

frequency drive

strategies

Just with PWM in DC motors, students may need help to understand how induc-tion motors are controlled using pseudo-sine waveforms and FET based circuits. and how FETs can provide a 48V peak-to-peak waveform from a 24V supply.

The DC1 output is used to trigger the oscilloscope to view the waveform of the single-phase output A signal . Use an external ‘scope for this, but not a PC-based device. The internal oscilloscope is not good for this as the sampling frequency is too low. However, it is a really useful tool for looking at the current flowing in the motor. The final task, examining the waveforms of signals A and B is left as an investigation for more able students.

40 - 50 mins

Instructor Guide

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16 Three-phase


This worksheet builds on student’s knowledge of single-phase motors to three-phase induction motors.

The investigation should show that three-phase motors are more efficient in equivalent situations. They have a number of other advantages over single-phase induction motors, such as uniformity of torque, smaller size/weight and self-starting. The students could be set the task of producing a presentation con-trasting the properties of three-phase and single-phase motors.

40 - 50 mins

17 Three-phase motor drive strategies

This is quite a lengthy worksheet, which aims to help students to understand how three FET-based circuits provide a three-phase driving signal.

As a simplification, resistors are used as a load for the three-phase power supply. This makes it much easier to study the driving waveforms. When the inductive load of the three-phase motor is connected, the waveforms are much more com-plicated and difficult to interpret.

Students might find the concepts of open circuit and floating outputs difficult to understand. The instructor may need to spend some time with the system diagram, describing to the students the waveforms they are likely to see on the oscilloscope. Understanding these waveforms will help students to understand what to look for when testing three-phase speed control circuits.

40 - 50 mins

18 Understanding

slip

The instructor may need to introduce this exercise with an explanation of the meaning of synchronous speed and slip.

They need to be confident in their ability to relate a driving frequency in Hz into a speed of rotation in RPM and vice-versa. The issue of the number of poles (and pairs of poles) in a motor is potentially confusing and will probably require input from the instructor. The diagrams in the Reference section, pages 56 to 59, will help to resolve this.

40 - 50 mins

Instructor Guide

Modern electrical

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19 Brushless DC

motor

This worksheet introduces the student to the permanent magnet three-phase synchronous motors (PMSM). This type of motor and the similar brushless DC motor (BLDC) are gaining popularity in various industries e.g. electrical vehicle motor/generators.

This motor cannot achieve synchronous speed instantaneously. It is started at low speed, where the permanent magnets can move and keep up with the ro-tating magnetic field. The frequency is then increased slowly until the motor is running at full synchronous speed.

This worksheet focuses more on understanding how the Brushless motor works and on the connections to it than on the significance of the speed/torque graphs.

Its hard to show this well as the driving waveform is dictated by the position of the Hall sensors and showing all this at one time is difficult.

However, as more and more applications use Brushless motors, students need to be familiar with it.

40 - 50 mins

20 Three-phase

generator

This worksheet introduces the permanent magnet three-phase synchronous gen-erator (PMG). This is in fact the Brushless DC motor studied in the previous work-sheet.

Typically, this dual motor/generator functionality is this motor’s strength. In electric vehicles, it can be used to provide the rotational energy to drive the wheels and act as a generator to charge the battery, under regenerative braking.

Note - three diodes are included in the kit so that you can build a full three-phase rectification system from the output of the Brushless DC motor.

40 - 50 mins

Instructor Guide

Modern electrical

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Equipment checklist

To deliver this set of worksheets you will need:

If you intend to use an external meter, please note that it needs to be a true RMS meter.

Code Description Qty

COM7414 RS232 lead 1

EM1100 Red 100cm lead 3

EM1125 Black 100cm lead 3

EM1150 Green 100cm Lead 2

EM1175 Yellow 100cm lead 2

EM1190 Blue 100cm lead 2

EM2159 Dyno 1

EM2391 3 phase motor 1

EM5337 Brushless motor 1

EM6066 Control box 1

EM6574 DC motor assembly 1

EM7432 Series motor 1

EM8614 Single phase motor 1

EM9856 Shunt motor 1

HP2045 Shallow tray 2

HP3701 Mains lead 1

HP3844 Foam 5

HP4039 Lids 6

HP5540 Deep tray 3

HP6002 Cable for brushless motor 1

HP6529 Binding post 2

HP6640 Extra deep tray 1

HPUSB USB lead 1

LK2346 Bulb 3

LK5203 10Kohm resistor 3

LK5243 Diode 1A 3

LK5250 link 5

LK5291 Lampholder 3

LK5297 Lead - black 4mm to 4mm unshroud-ed 1

LK5298 Lead - red 4mm to 4mm unshrouded 3

LK8900 baseboard 1

XXXXX Locktronics foam insert 1

Modern electrical

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Troubleshooting

Dynamometer arm sticking

Problem: On newer models the Dynamometer

arm can stick in the powder coating of the load

cell. This affects readings at low torque.

Remedy: put a little WD40 on the load cell so

that the arm does not stick

Problem: comms errors.

Solution: some lap tops seem to reduce the

USB response speed as a battery saving meas-

ure .IN Task manager set the SCADA app as a

higher priority.

Modern electrical

machines


Version control

04 07 19 first release

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